Glucocorticoid Insensitivity at the Hypoxic Blood–Brain Barrier Can Be Reversed by Inhibition of the Proteasome

BACKGROUND AND PURPOSE: Glucocorticoids potently stabilize the blood-brain barrier and ameliorate tissue edema in certain neoplastic and inflammatory disorders of the central nervous system, but they are largely ineffective in patients with acute ischemic stroke. The reasons for this discrepancy are unresolved. METHODS: To address the molecular basis for the paradox unresponsiveness of the blood-brain barrier during hypoxia, we used murine brain microvascular endothelial cells exposed to O(2)/glucose deprivation as an in vitro model. In an in vivo approach, mice were subjected to transient middle cerebral artery occlusion to induce brain infarctions. Blood-brain barrier damage and edema formation were chosen as surrogate markers of glucocorticoid sensitivity in the presence or absence of proteasome inhibitors. RESULTS: O(2)/glucose deprivation reduced the expression of tight junction proteins and transendothelial resistance in murine brain microvascular endothelial cells in vitro. Dexamethasone treatment failed to reverse these effects during hypoxia. Proteasome-dependent degradation of the glucocorticoid receptor impaired glucocorticoid receptor transactivation thereby preventing physiological glucocorticoid activity. Inhibition of the proteasome, however, fully restored the blood-brain barrier stabilizing properties of glucocorticoid during O(2)/glucose deprivation. Importantly, mice treated with the proteasome inhibitor Bortezomib in combination with steroids several hours after stroke developed significantly less brain edema and functional deficits, whereas respective monotherapies were ineffective. CONCLUSIONS: We for the first time show that inhibition of the proteasome can overcome glucocorticoid resistance at the hypoxic blood-brain barrier. Hence, combined treatment strategies may help to combat stroke-induced brain edema formation in the future and prevent secondary clinical deterioration.